Fix overflow issue in TcgProtocol

Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by  : Yao Jiewen <jiewen.yao@intel.com>
Reviewed-by  : Dong Guo   <guo.dong@intel.com>


git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@14396 6f19259b-4bc3-4df7-8a09-765794883524